Vanishingly few things show up in "pure form", anywhere in nature. Gold is is all mixed up into quartz, save for flakes in riverbeds. Nitrogen and argon, famously unreactive, nonetheless come mixed with the other, with oxygen mixed in besides.

Diamond, almost pure carbon, is the only that comes to mind.

So, it is meaningless to single out lanthanides for this. What does distinguish them, instead, is that they are expensive to separate from one another. In certain places such as Yterby and the site mentioned in the original article, ore contains a concentrated mix of many lanthanide compounds. It remains a chore to get the praseodymium and the neodymium into separate ingots.